California Association of Chemistry Teachers
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Robert 8. Fischer California State College, Dominguer Hills Dominguez Hills, California 90747
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Ion-Selective Electrodes
The first ion-selective electrode was the glass membrane electrode. For many years it was the only practical one. The possibility of using an electrode of this type for the measurement of p H was recognized a t least sixty years ago. The possibility became a practical reality during the decade of the 1930's with the development of electronic voltmeters with sufficiently high input resistance characteristics. It is only much more recently, however, that the functioning of a glass membrane electrode could he explained other than on a highly empirical basis. Alone with an imnroved. less emnirical understanding of the ;se of the glass electrode for ;H measurement has come the development of glass electrodes far the measurements of a few &her ions and, of particular importance in the last decade or so, the introduction and use of several other types of ion-selective electrodes. In this paper we will consider hriefly the composition, functioning and analytical applicability of three major types of ion-selective membrane electrodes, the glass, solid state, and liquid ion-exchanger types. We will not discuss certain others, such as enzyme electrodes, which are quite similar to these types. We will not attempt to tabulate the electrodes which are now commercially available, and we will not survey the many specific applications which have been reported in the literature.' The mechanism of the glass electrode will he described first, because it provides a basis for the consideration of the others.
means of an electronic voltmeter, the read-out device of which is commonlv calibrated directlv in oH units. The discussions in textbooks of the 'functioning of the elass memhrane electrode for DH measurement have frequently been based upon the postulate that hydrogen ions selectively flow through the glass memhrane from the more concentrated to the less concentrated solution. In some cases these explanations have been stated with a high degree of finality even though this never has been really substantiated. This type of explanation may he illustrated by the following quotations from textbooks: ". . . a membrane, which is permeable only to hydrogen and hydroxyl ions and water . . ." (1955); "hydrogen ions can move through the glass wall" (1957); "the glass electrode is a memhrane that is selectively permeable only to hydrogen ions-that is, it allows hydrogen ions to diffuse freely, but does not permit the passage of other cations or anions" (1963); "the essential part of the glass electrode is a very thin glass memhrane which is permeable only to hydrogen ions; the pores of the glass are too small to per&t the of otber cations o;anions3' (1966). Other authors of textbooks, recognizing that any stated mechanism based upon the of hydrogen ions through the membrane was either unsubstantiated or overly simplistic, have referred to the glass memhrane as "pH-responsive" or "pH-sensitive" and have then proceeded on an empirical basis to quantitative statement and discussion of the observable relationship between p H and potential. Whatever the suggested physical or chemical mechanism of the electrode response may he, the fact has long been recognized that the response of a glass memhrane electrode is describable by means of a common type of Nernst equation. A ten-fold difference in hydrogen-ion concentration (activity) corresponds to a potential difference of 59 mV, under a wide range of commonly encountered experimental conditions. Relatively recently, a series of experiments was conducted using" tritium-labelled hvdroeen ions to ascertain . whether or not any hydrogen ions in fact do pass through the glass memhrane. It was definitely concluded that no ions are transported through the memhrane, not even when attempts were made to force them through . by . electrolvsis. L& us now describe hriefly the common view today of the functioning of the glass membrane electrode. The chemical composition of a typical glass may be represented as NazO Alz03. SiOz. Considering only the SiOz portion for a moment, the glass consists of a random, three-
Glass Electrode for pH Measurement A common form of the glass memhrane electrode is depicted in Figure 1. In use for measurement of pH, the glass memhrane separates two liquid phases. One is inside the electrode as shown and is typically 0.1 F hydrochloric acid. The other is the test solution into which the electrode is dipped. Two reference electrodes are employed, one in each of the two solutions. The difference in potential between the two reference electrodes is measured by shielded l e a d 4 1
.
9-AgCi
electrode
O.IF HCI Figure 1 .
trode.
Glass
membrane elec-
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Oxygen
~ i g u r e2. Diagram of sioI tetra-
hedra.
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Presented at the March, 1973 meeting of the California Association ~ ~of Chemistry Teachers. Southern Section. 'More detailed information on all these topics is readily available in the regular scientific journals and in materials provided by the manufacturers of ion-selective electrodes. One especially valuable collection of articles on the basic principles and areas of application is contained in "Special ~ublication314" of the ~ a t i o n a l ~ u r e aofu Standards, (Editor: Durst, R. A,) 1969. Volurne51, Number 6. June 1974 / 387
dimensional network of SiOn tetrahedra, represented in two dimensions in Fieure 2. Generallv each silicon atom is bonded to four oxygen atoms, and kach oxygen atom to two silicons. Here and there, however, are some oxygen atoms which are not completely bonded to silicon atoms. Wherever this condition occurs, a negative "site" exists on the unhanded oxveen. .- , and a "hole" or void exists in the space that would otherwise be occupied by a silicon atom. Throughout the elass. then. considerine the SiOz portion. there a r e negat&e sites ahd holes. 1; an acttiai glass; manv of these holes are occupied by sodium ions, thus acof the chemical formula, counting for the NazO but each sodium ion is not necessarily bound to a particular location. Therefore, the glass includes some negative sites on partially bonded oxygen atoms, along with sodium ions which are sort of floating around within the structure. A glass membrane electrode does not function properly unless or until i t has been soaked in water. Some physical swelling occurs during this soaking. Presumably the aqueous medium penetrates the glass, forming a hydrated layer at each of the interfaces of the glass memhrane, as represented in Figure 3. The hydrated layers are frequent-
n.1 F Hcl
I inner
I I dry
:hydrated laver
glass
laver
outer
:
hydrated iavnr
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glass membrane Figure 3. Representation of hydrated glass membrane.
ly a few hundred angstroms thick, while the dry glass layer may be of the order of 50 pm. Cation exchange occurs within the inner and outer hydrated layers, between sodium ions from the glass and hydrogen ions from the inner or outer solution ~a+,l,,, + H+,,,I.,~,. == Nac,.,.,i,, + H+,I:,,, The hydrogen ions occupy some of the negative. sites in the hydrated layers. The quantity of these exchanged hydrogen ions is proportional to the activity of hydrogen ions in the adjacent solution, and the result is the establishing of a phase boundary potential at each of the two hydrated layers. In order for the difference between these two phaseboundary potentials to he measurable, there must be some electrical conductivity through the dry glass layer. This presumably consists of ionic conduction involving the sodium ions which can move about interstitially. It is not a t all necessary that a given sodium ion move all the way through the dry glass layer, but rather that many ions move short distances each to provide the net ionic conductivity. Thus, the functioning of a glass memhrane electrode for p H measurement is now commonly considered to involve (a) the existence of hydrated glass layers a t each of the glass-solution interfaces, (b) cation exchange within each of these interfaces between sodium ions from the glass and hydrogen ions from the solution, and (c) ionic conductivity involving interstitial cations through the dry glass layer. In all sincerity we should recognize that this explanation, while better substantiated than those proffered in earlier years, should he considered to he only as complete and final as the extent to which we are willing to accept i t without asking further questions. This comment is of general applicability to other scientific explanations as well.
Glass Electrodes for Measurements of Ions Other than Hydrogen Ion The selectivity of the glass memhrane electrode for hydmgen ion is dependent upon a preference for hydrogen 388
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Journal of Chemical Education
ion in the cation-exchange reaction. This preference arises from the weakly acidic character of the negative sites which exist at t h e incompletely bonded oxygen atoms. The selectivity is not perfect, of course. If there are relatively few hydrogen ions compared to some other cation, much or most of the exchange can be with the other ion. For example, in a solution of high pH, the pH reading comes out to be too low (that is, the apparent concentration of hydrogen ion is too high), simply because of appreciable exchange with cations which are present in significantly higher concentration than hydrogen ion. The degree of selectivity in favor of hydrogen ion is appreciably dependent upon the composition of the glass. For example, if some of the fully bonded silicon atoms in the tetrahedral configuration represented in Figure 2 are replaced by aluminum atoms, the negative charges of the four adjacent oxygen atoms are offset by three positive charges rather than four. Thus the negative sites upon incomnletelv bonded oxveen atoms are relativelv even more negative, a n d the preiLrence for exchange with hydrogen ion is diminished. This can result, in fact. in a glass membrane electrode which is quantitatively r&ponsive to some cation other than hydrogen ion. A sodium-ion glass electrode is one in which the glass membrane is relatively high in aluminum content. Solid-state Electrodes Glass is not the only substance which can he formed into membranes and which can exhibit both cation exchange with an adjacent solution phase and ionic conduction within. Let us consider now what is known as the solid-state type of ion-selective membrane electrode. Polycrystalline or Precipitate Solid-state Electrodes One early form of solid-state electrode was developed and studied in the author's laboratory in the 1950's. The electrode consisted of a membrane of a precipitated substance, such as barium sulfate, embedded in a matrix of paraffin mounted on the end of a 1.0-cm diameter glass tube. Presumably the paraffin does not "wet" the surfaces of the inorganic crystals, so a network of tiny pores or channels exists throughout the membrane at the crystalparaffin boundaries. When each of the surfaces of the membrane is in contact with an aqueous solution. the solution tends to penetrate the membrane and ions are absorbed onto the surfaces of the crystals. Selectivity in the primary adsorption layer is as described by the PanethFajans-Hahn rule and other relevant factors, while electrical conduction through the membrane involves the oppositely charged counter ions. Although perfect selectivity and Nernstian response were not achieved with these membrane electrodes, i t was possible to correlate potential with solution composition and even to use these electrodes in potentiometric titrations under some conditions. The diagram of Figure 4 is generally applicable to all solid-state membrane electrodes. The most extensive practical development to date of the polycrystalline or precipitate type of solid-state electrode has been accom-
Figure 4. Solid-slate membrane electrode
plished with silicone rubber as the embedding matrix. These membranes are typically formed by mixing the precipitated particles, usually 5-10 pm in size, with the silicone monomer, followed by polymerization. Electrodes of this type are commercially available with a number of different precipitated substances, each with its own distinctive selectivity. A membrane of this type presumably exhibits preferential adsorption onto the surfaces of the tiny particles of some ion which is in common with the precipitate. Thus, for example, an electrode with a memhrane of particles of silver iodide in a silicone rubber matrix is ion-selective for the iodide ion. This particular electrode is analogous, insofar as the potentiometric measurement of iodide is concerned, to the silver iodide coated, silver wire electrode. However. the latter is susceptible to interference from other redbx systems, while the memhrane electrode is not. Membrane electrodes with other precipitated particles are ion-selective for other ions. Interestingly enough, a silver iodide solid-state electrode can be changed into a cyanide-indicating electrode, by soaking for a time in a cyanide solution. Presumably the surface layers of the particles from silver iodide are converted to silver cyanide. Single-Crystal Solid-state Electrodes Consider next the type of electrode in which the membrane consists of a thin layer of a single crystal of silver sulfate, again with reference to Figure 4. The lattice structure and the relative sizes of the silver and sulfide ions are such that there are some "positive holes," that is, some voids where silver ions are missing from what would otherwise he a complete, perfect crystalline lattice. There is correspondingly some freedom of movement of silver ions within the lattice. The net consequence is a silver ion indicating electrode, with exchange of silver ions across the interfaces between the memhrane and the inner and outer solutions and ionic conduction through the thin single crystal. Other practical examples of useful single crystal solidstate membrane electrodes are the iodide and the fluoride indicating electrodes, in which the memhrane consists of a thin crystal of silver iodide or of lithium fluoride, respectively. In the latter, the crystal structure consists of alternate layers of lithium and fluoride ions, with some holes in the fluoride layer. The overall ionic movement consists of many tiny movements, in each of which a fluoride ion shifts its position into an adjacent hole. The lanthanum fluoride can be "doped" with a little europium(II), which seems to lower the electrical resistance and to aid the ionic transport which is essential to making potentiometric measurement with reasonably rapid response. Liquid Ion-Exchanger Membrane Electrodes Most of our discussion thus far has involved the ion exchange of monovalent ions. For a divalent ion to undergo exchange requires in effect that there be two exchange sites located in suitable spatial relationship to each other. A solid-state electrode has, of course, many available exchange sites, but all are in fixed locations that hardly can he expected to match up in pairs with calcium ions, for example. However, if the membrane consists not of solid, rigidly situated particles but rather of liquid ion exchanger, the exchange sites are free to move about. The liquid ion-exchanger type of ion-selective electrode is depicted in Figure 5. The memhrane is not self-supporting, as is a solid memhrane, but i t can readily be held in place by a thin porous layer of glass or plastic. As with other types of membrane electrodes, a standard solution is used internally, the test sample forms the outer solution, and the potential is measured between two reference electrodes, one of which is in each of the two solutions. By the time this type of electrode was investigated, much information
LIOUIOMCMBPAht O1.C L I C I R O D E
Figure 5 . Liquid ion-exchanger membrane electrode.
had already been gathered on various types of ion-exchange materials and their selectivities for various ions. This backlog of data on liquid ion-exchangers was directly applicable to the development of useful ion-selective electrodes, particularly for multivalent ions. A number of electrodes of this type are now commercially available. Analytical Applicability Measurement of Thermodynamic Activity The magnitude of the electrical potential developed by an ion-selective electrode is a function of the thermodynamic activity of the measured species of ion, not of its analytical concentration. The quantitative relationship between the potential E and the activity a may he expressed by means of a Nemst equation
RT
E = k + - InF na The quantity k is a function of the reference electrodes, the internal solution, and any asymmetry exhibited by the membrane itself. The other symbols have their usual meanings. With appropriate or typical values for k , R, T, and F, and with an n value of 1, there is a 59 mV potential difference for a ten-fold difference in ion activity. Measurement of Analytical Concentration The thermodynamic activity is directly the quantity of interest in numerous applications of ion-selective electrodes. In many situations, however, it is the analytical concentration. rather than the activitv, which is the zoal of the measurement. The distinction-between concesration and activity involves such practical considerations as ionic strength of the solution, the presence of constituents which can form complexes with the substance of interest, and the DH if the substance exhibits weak acid or base characteristics. With calibration standards that are appropriate for the particular test sample, it is possible to-make measurement of concentration by means of ion-selective electrodes. The one requirement is that the calibration standards he comparable to the test solution in those factors which cause activity to differ from concentration. One common technique is to add to each solution, standard and unknown alike, sufficient electrolyte to swamp out any variations that would otherwise exist in ionic strength. A special reagent called TISAB (total ionic strength adjustment buffer) has been developed for the determination of fluoride in drinking water; it includes an aluminum or iron salt to provide constant and reproducible complexing of certain normal constituents in the water, buffering ingredients to adjust the p H to 5-6, and other salts to provide a high ionic strength. The potential generally differs somewhat less than 59 mV for a ten-fold difference in concentration for a singlycharged ion. So it is often desirable to prepare an empirical calibration line or curve bv measurements of several standard solutions of the substance of interest. In the standard addition method, measurement is made Volume51, Number 6, June 1974
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upon the unknown test solution, then a known increment of the species of interest is added and the potential is measured again. The results of the two measurements may he written in two Nernst equations, from which the concentration (activity) of the unknown may he calculated. An empirical calculation may be made graphically. This method assumes, in effect, that the added increment does not significantly change the ionic strength or the degree of complexing within the test solution. An analogous technique is the method of standard deductions, in which a known increment of the unknown species is removed, by precipitation for example, between the two readings of electrical potential. Selectivity The selectivity of the response of an ion-selective electrode may be expressed for any particular situation by means of a selectivity ratio, which is simply the ratio of the millivolt responses to equal changes in the activities or concentrations of the two substances. For example, the selectivity ratio of a polycrystalline silver iodide electrode is typically about 2 x 102 for iodide over bromide, and 3 x lo7 for iodide over sulfate. Thus this iodideindicating electrode responds slightly (about one two-hundredth as much) to bromide as to iodide, while there is virtually no interference by sulfate in the measurement of iodide ion. To take another example, the selectivity ratio of a single crystal lanthanum fluoride electrode for measurement of fluoride is of the order of lo3 or higher over most other anions but is only lo1 over hydroxyl ion. Accordingly, buffering is generally employed in fluoride determinations to minimize interference from hydroxyl ion. The preceding examples serve to illustrate two considerations. First, the selectivity of ion-selective electrodes is fully adequate for many analytical applications. Second, reasonably simple adjustments of the overall composition of the test solution often suffice to enswe adequate selectivity in those instances in which the electrode itself is subject to interference. Precision The precision necessary in the measurement of the electrical potential of an ion-selective electrode is governed by the desired precision in the result of the analytical determination. Consider, for example, that a precision of &I% is desired in the measurement of the activity of a singlycharged ion. It may he shown, by inserting in turn values of (1.00 a) and of either (0.99 a ) or (1.01 a ) into the Nernst equation, that a precision of *0.2 mV is required in the measurement of E. This reauirement is much more stringent than is typically encountered in p H measurements and in most classical aonlications of other tvoes .. of potentiometric indicating elec&ode systems. The rapid development in recent years of various types of ion-selective electrodes has been accompanied by the development of much more stable reference electrodes and more orecise electronic voltmeters than those which had been used successfully for other purposes. Because of the desired high precision in measurements of electrical potential in many analytical determinations, the temoerature factor in the Nernst relations hi^ is critical. It is interesting to note, however, that if the internal
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solution and the reference electrodes are rhosen such that the overall E comer out to he at or clwe to zero with the partirular test solution 111 interest, the significance of the temperature tkcror is minimized. The accuracv of a measurement of electrical uorential is, of course, not synonymous with its precision. The constancy of the electronic voltmeter and the accuracy of its calibration may render its accuracy far less than its shortterm precision. Furthermore, the constancy and accuracy of the standard solutions which are used for calibrating the overall system influence the accuracy of the result of an analytical determination. Sensitivity The sensitivity of an analytical procedure is limited by the selectivity and precision factors. In addition, the sensitivitv mav he limited bv the finite solubilitv of the membrane in the test solution. For example, there is inevitably some dissolution of silver iodide from a solid-state electrode containing that substance into any solution brought into contact with it. It is not possible to detect and to measure a concentration of iodide any lower than that put into the solution by the membrane itself. Response Time An ion-selective electrode should be soaked prior to use, as described in our discussion of the glass electrode. From then on, the response time depends largely upon the magnitude of the variations in going from one test solution to another. Typical response times are of the order of seconds. One of the important areas of application of ion-selective electrodes is in the continuous monitoring and control of flowing systems. The changes in concentraiion or activity of the constituent being measured are often only slight, and virtually instantaneous response time is achieved. Lifetime of Electrode A solid-state memhrane electrode is gradually consumed during use, because of the slight solubility of its crystalline component. A typical lifetime is of the order of a few months if the electrode is in continual use in fresh solutions (that is, solutions which are unsaturated with respect to the memhrane component). Under more common conditions an electrode is used only intermittently with fresh test solutions, and the lifetime of the electrode is much longer. An electrode of the liquid ion-exchanger type is also subject to dissolution into the test solutions with which it is in contact. However, it is a simple matter to add more of the exchaneer from time to time as needed. As shown in Figure 5, some electrodes are designed with a reservoir of the ion exchanger, and can be "recharged" as desired. Use of Nonaqueous Media Although most applications of ion-selective electrodes to date have involved aqueous media, a number of useful apolications have been develo~edwith nonaaueous test solutions. Empirical calibratiou'is necessary; analytical results are reproducible with good quantitative precision and accuracy.
